This invention relates to settable compositions which can encapsulate hazardous waste materials and relates particularly, though not exclusively, to a method of encapsulating hazardous waste materials including heavy metals such as arsenic, nickel, chromium residues and mercury as well as radioactive materials.
Arsenic and arsenic containing compositions have been widely used in Australia as sheep and cattle dips, and have also been used in pesticides. Mercury and mercury containing compositions have also been widely used in Australia and other countries. With the phasing out of arsenic and mercury compounds (due to their toxicity), a large stockpile of waste arsenic and mercury components exists.
Organo nickel and chromium, and nickel and chromium containing compositions are widely used in Australia and other countries. Metal plating and anodising processes use these compounds, and the processes generate waste nickel and chromium residues which are quite concentrated and are stored in drums. The residue is hazardous and toxic and a large stockpile of waste nickel and chromium components exists.
The above mentioned hazardous waste materials and toxic components are housed in drums which have a limited life. The components in the drum are typically in the form of a contaminated liquid or sludge which is almost impossible to safely encapsulate. The sludge contains a variety of contaminants such as rust from the drum, waste particles, solids, and a variety of liquids.
Radioactive materials and components are also hazardous substances. Apart from their use as fuel for nuclear reactors, radioactive materials also have medical applications and other industrial applications. For example, radioisotopes are used in the medical field for the diagnosis and treatment of various types of illness. In some industries, for example in the mining industry, a radioactive source is used in certain types of instruments for measuring, for example, the thickness of materials. However, one of the problems associated with using radioactive materials is finding a socially and environmentally acceptable method of disposing of the radioactive waste materials. Various proposals have been put forward for encasement or encapsulation of radioactive materials which can than be safely disposed of by burial in uninhabited regions of the Earth.
Attempts to encapsulate hazardous waste materials in concrete have met with limited success as concrete and cement do not bind well in the presence of contamination. However, concrete or concrete like products are ideal for encapsulation as the concrete is hard, has a very long life, and can be moulded prior to setting.
The present invention is directed to a method by which hazardous waste materials or components thereof can be encapsulated, even if the materials are contaminated with other impurities.
According to one aspect of the present invention there is provided a method of encapsulating a hazardous waste material or component thereof, the method comprising: adding the hazardous waste material to a settable composition, the composition comprising a calcium carbonate and a caustic magnesium oxide, forming a slurry, and allowing the slurry to set to encapsulate the waste material or components thereof.
According to another aspect of the invention there is provided a method of encapsulating an arsenic component selected from the group consisting of arsenic, sodium arsenite, arsenic trioxide or arsenic pentoxide, the method comprising:
adding to the arsenic component a sulphate, an iron chloride and/or an alkaline agent, and water to form a slurry, mixing the slurry with a settable composition, the composition comprising a calcium carbonate and a caustic magnesium oxide, and allowing the composition to set to encapsulate the arsenic component.
It is found that when encapsulated in this matter, very little arsenic is leached out of the set composition which has a concrete like appearance.
Indeed, leach rates are less that the allowed leach rate of 5.00 ppm arsenic. The sulphate which is added to the slurry is preferably an aluminium sulphate. The alkaline agent is preferably a carbonate such as calcium carbonate. The iron chloride can comprise ferric chloride.
Suitable ranges include:
Arsenic componentxe2x80x94100 weight units (e.g. grams),
Sulphatexe2x80x9410-80 weight units, more preferably about 50 weight units,
Alkaline agent (e.g. calcium carbonate)xe2x80x9410-80 weight units.
Ferric chloride (if present)xe2x80x945-50 weight units.
To form a slurry 100-500 weight units of water per 100 weight units of arsenic component can be added.
According to a further aspect of the invention there is provided a method of encapsulating mercury or components thereof, the method comprising: adding the mercury or components thereof to a settable composition, the settable composition comprising a calcium carbonate and a caustic magnesium oxide, forming a slurry, and allowing the slurry to set to encapsulate the mercury or components thereof.
It is found that when encapsulated in this matter, very little mercury is leached out of the set composition which has a concrete like appearance. Indeed, leach rates are less than the allowed leach rate of 5.00 ppm mercury.
The mercury waste is typically stored as a sludge. One source of mercury waste is from Clarriflocculator sludge, or Brine sludge. This sludge contains between 100-200 mg of mercury per kilogram of sludge, as well as water, dust/soil, and other impurities, which makes encapsulation in concrete unsatisfactory.
Suitable ranges for the method include:
Mercury containing sludgexe2x80x941000 grams,
Settable compositionxe2x80x941000 grams,
Waterxe2x80x94270 ml.
Additivesxe2x80x94100 grams.
According to a still further aspect of the invention there is provided a method of encapsulating nickel. and chromium or components thereof, the method comprising:
adding the nickel and chromium or components thereof to a settable composition, the settable composition comprising a calcium carbonate and a caustic magnesium oxide, forming a slurry, and allowing the slurry to set to encapsulate the nickel and chromium and components thereof.
It is found that when encapsulated in this matter, very little nickel and chromium is leached out of the set composition which has a concrete like appearance. Indeed, leach rates are less than the allowed leach rate of 0.2 ppm nickel and chromium.
The nickel and chromium waste is typically stored as a thick liquid or sludge. One source of nickel and chromium waste is from the metal plating and anodising industry. This liquid can contain between 10 mg-28,000(?)mg of nickel and about 10 mg-200,000 mg chromium per kilogram of liquid, as well as water, dust/soil, and other impurities, which makes encapsulation in concrete unsatisfactory.
If desired, a filler can be used. The filler can include an ash, but other fillers are envisaged. The filler can comprise between 10-90% of the set composition, and more preferably between 40-60%
Suitable non-limiting ranges for the method include:
Nickel and chromium containing liquidxe2x80x94150 ml,
Settable compositionxe2x80x94300 grams,
Waterxe2x80x94400 ml.
Additivesxe2x80x94100 grams.
According to yet another aspect of the invention there is provided a method of encapsulating radioactive materials, the method comprising: adding the radioactive materials to a settable composition, the settable composition comprising a calcium carbonate and a caustic magnesium oxide, forming a slurry, and allowing the slurry to set to encapsulate the radioactive material therein.
It is found that when encapsulated in this matter, virtually no radioactive material is leached out of the set composition which has a concrete-like appearance. Furthermore, radioactivity levels of the leached material are well below the background radiation level.
The method of encapsulation according to the invention is particularly suited to low to mid level radioactive material, for example, monazite. Preferably, the radioactive material is added to the settable composition in the form of a powder. Preferably the radioactive material or components thereof are ground to a particle size in the range from 0.01 xcexcm to 5.0 mm, more preferably the particle size falls within the range 0.1 xcexcm to 1.0 mm.
Preferably the settable composition further comprises lead or a lead compound such as, for example, lead oxide. It is thought that the lead in the composition assists in absorbing the radiation from the radioactive material.
Suitable non-limiting ranges for the method include:
Radioactive materialxe2x80x941000 grams,
Settable compositionxe2x80x94700 to 2200 grams,
Lead oxidexe2x80x94300 to 1500 grams
Waterxe2x80x94500 to 900 ml.
Additivesxe2x80x94250 to 375 grams.
The term caustic magnesium oxide includes a magnesium composition which comprises magnesium carbonate and a decarbonated magnesium. The term also covers a magnesium carbonate which has been treated, for instance, by heating, to liberate carbon dioxide, thereby forming a composition which is partially calcined. The exact structure of the composition and of the caustic magnesium oxide is not known, but the term will be used to include the structure formed by heating magnesium carbonate to partially decarbonate it, especially at the temperature ranges described.
The composition of calcium carbonate and caustic magnesium oxide can be formed by treating dolomite. Dolomite is a calcium magnesium carbonate found in nature. A true dolomite comprises about 54% calcium carbonate and 43% magnesium carbonate. Natural dolomites contain impurities of various differing types which can include alumina, iron and silica.
The percentage of the calcium and magnesium carbonate can vary in dolomites. For instance, dolomite containing 65% calcium carbonate and 30% magnesium carbonate is called a low magnesium dolomite. Conversely, a dolomite containing 60% magnesium carbonate and 30% calcium carbonate is called a high magnesium dolomite.
It is found that heating dolomite will cause carbon dioxide to be liberated, and the rate of liberation of carbon dioxide can be controlled and varied to provide fully or partially calcined dolomites.
If the dolomite is heated at 1,500xc2x0 C., all the carbonate is liberated as carbon dioxide and a mixture of calcium oxide and magnesium oxide is left. These oxides are well known as for use in refractory material, but the oxides are not suitable for a cementitious material.
If dolomite is heated at a lower temperature, not all of the carbonate decomposes to liberate carbon dioxide. Indeed, it is noted that the heating can be controlled such that the magnesium carbonate preferentially releases carbon dioxide over the calcium carbonate.
Thus, heating at a temperature range of typically between 500-800xc2x0 C. will cause preferential decomposition of the magnesium carbonate.
By controlling the preferential decomposition, dolomite can be treated to form a settable composition by the dolomite into a composition comprising a caustic magnesium oxide.
The preferential decomposition of dolomite can be enhanced by additives such as inorganic salts. A suitable salt is a metal sulphate such as aluminium sulphate or magnesium sulphate which can be added from 0.1%-5% prior to heating. The salt appears to preferentially decrease the decarbonisation temperature of MgCO3 without substantially affecting the higher decarbonisation temperature of CaCO3. The salt can increase the differential temperature from 100xc2x0 C. to 200xc2x0 C.
Suitably, the caustic magnesium oxide has between 0.1%-50% of the carbon dioxide retained within the magnesium carbonate, and preferably between 23%-28%.
While the molecular structure may be difficult to envisage, the structure may comprise a mix of calcium carbonate, magnesium oxide, and magnesium carbonate. The amount of carbon dioxide retained in the composition has an effect on various parameters such as hardness, and setting rate. Between 20%-30% retained carbon dioxide offers a suitable set rate for many applications. Increasing the amount of carbon dioxide decreases the set rate, and decreasing the amount of carbon dioxide increases the set rate.
The composition can also be prepared synthetically by mixing or blending calcium carbonate with preformed caustic magnesium oxide. In this variation, the caustic magnesium oxide can be prepared by subjecting heat to partially drive off carbon dioxide until the desired level of calcination is obtained.
In a further variation, a natural dolomite may be heated in the manner described above to provide a composition comprising calcium carbonate and caustic magnesium oxide, and if the natural dolomite is magnesium deficient (for instance, a low magnesium dolomite), additional caustic magnesium oxide can be added to the mixture.
For example, a low magnesium dolomite ore containing 65% calcium carbonate and 30% magnesium carbonate plus impurities can be calcined so that the magnesium converts to partially calcined caustic magnesium oxide but essentially where between 2%-20% of the original entrained carbon dioxide within the magnesium is retrained.
By being able to add caustic magnesium oxide, and calcium carbonate, and being able to vary the blend of the two, it is possible to provide compositions for use as cement having any required predetermined weight or percentage of the blended materials.
The particle size of the composition can be varied if desired. A suitable particle size of 50-70 micron with 90% passing through a 60 micron sieve allows the composition to be used in a variety of applications. The composition can be ground to the particle size if required and this can be done before or after treatment. Other particle size ranges are also envisaged such as from 10-1000 microns.
A range of 10%-90% caustic magnesium oxide and 90%-10% calcium carbonate can be used, with a preferred mix being 60%-70% magnesium and 30%-40% calcium.
For instance, one tonne of dolomite will contain 650 kilograms (kg) of calcium carbonate (CaCO3) and 300 kg of magnesium carbonate (MgCO3) plus 5% impurities. The magnesium carbonate will contain 156:57 kg of CO2. When 95% of this CO2 is removed the weight loss will be 148:74 kg. The calcined weight of the dolomite will now be 851:26 kg which will include 650 kg of calcium carbonate plus 143.3 kg of magnesia oxide and 50 kg of impurities. (CaCO3650 kg/MgO 143:43 kg+7:8285 kg+impurities 50 kg=851.26.)
Dolomite 1000 kg=650 kg CaCO3 before calcination
300 kg MgCO3+50 kg impurities
After calcination=650 kg CaCO3 
151:258 kg caustic (MgO+7.8CO2)+desired weight of selected caustic magnesia oxide+impurities 50 kg
The composition can be formulated as a dry fine powder (that is similar to the Portland Cement powder).
Another source of the composition can be based on the calcined magnesites and dolomites directly from the magnesium industry. These are predominantly magnesium oxide (typically over 90%) with calcium oxide (ranging 3-18%) and containing a low amount of (0-5%) carbon dioxide. This commercial form of caustic magnesium oxide may even contain dead burnt calcium oxide or dead burnt magnesium oxide but is still useful in the encapsulation process although it is not specifically calcined for encapsulation.
To the composition can be added various additives. The additive or additives may accelerate the formation of strong binding agents, and may assist in the recrystallisation of the composition to make it set. In the setting process, various added fillers (which can include organic fillers, inorganic fillers, solid and liquid fillers, radioactive fillers, toxic fillers, and the like) can be trapped in the set matrix.
One additive can comprise a sulphate which may be added at rates of between 0.01% up to 20%, more typically 0.01% up to 10%. A suitable sulphate can comprise sulphuric acid, or a metal sulphate such as magnesium sulphate or aluminium sulphate.
Another desirable additive is one which act as a source of carbonation in the composition to assist in the setting process. A carbonate which can decompose or react to liberate carbon dioxide is preferred. One suitable additive can be a metal carbonate such as sodium carbonate. Another suitable additive can include a carboxylic or polycarboxylic acid which can react to liberate carbon dioxide. Another advantage of sodium carbonate is that it will carbonate any completely oxidised fillers which may be used (for instance coal ash).
Other additives may include citric acid, lemon acid, acetic acid, glycolic acid, oxalic acid, other di or poly carboxylic acids, or other acidifying agents. Possible substitutes for the citric acid include tartaric acid, Salicylic acid, ethylenediamine tetra acetic acid (EDTA) or other tetra acids. These additives may be added at between 0.01%-10%, more typically 0.01% to 5%. If the additives (such as citric acid or lemon acid) are solids, they are suitably pre-ground and powdered to enable them to be efficiently blended with the remainder of the composition. A grind size less than 250 mesh can be used. The aluminium sulphate may be commercially available aluminium sulphate having a hydration figure of 14. Of course, higher or lower hydrated aluminium sulphates can also be used with the appropriate weight adjustments.
Another acidifying agent may comprise sulphuric acid and this may be added to the water mixture in up to 5% by weight.
In a preferred feature, the additives include aluminium sulphate and a citric acid (or equivalent acid such as glycolic acid or acetic acid). Additionally, a salt such as sodium chloride can be provided.
The additives can be premixed and added to the composition. The amount of premix added can vary for instance from about 3%-10% or more. It appears that when fillers of small size (for example below 70 micron) are used, the amount of premix added should be larger (about 10%), while fillers of larger size allow less premix to be added (e.g. 3%-7%).
If the premix comprises (a) aluminium sulphate, (b) an organic acid and (c) a salt, it is preferred that (a) is present between 40%-80%; (b) is present between 10%-60% and (c) is present between 1%-20%.
While not wishing to be bound by theory, it appears that ingredient (a) provides early strength to the set composition, and may assist in the formation of brucite (Mg(OH)2), and a gelatinous polymer of aluminium hydroxide, both which help with initial bonding of the composition. It also appears that (a) provides water proofing properties.
Ingredient (b), for instance citric acid, appears to assist in the carbonisation of MgO and Mg(OH)2 to recrystallise the composition into a set material. The acid may also act as a ligand to form complexes around the fillers (for instance metal ions) helping to trap them in the setting or set matrix. The carbonisation process can continue over a long period of time which can provide long lasting strength to the set material. Ingredient (c) appears to assist in achieving an early strength to the composition.